Coatings of W-Co-Cr-C are used in those application where both superior wear and corrosion resistance are required. A typical composition for these coatings comprises about 8 to 10 weight percent cobalt, about 3 to 4 weight percent chromium, about 4.5 to 5.5 weight percent carbon and the balance tungsten. These coatings can be successfully applied to various substrates, e.g., iron base alloy substrates, using known thermal spray techniques. Such techniques include, for example, detonation gun (D-Gun) deposition as disclosed in U.S. Pat. Nos. 2,714,563 and 2,950,867, plasma arc spray as disclosed in U.S. Pat. Nos. 2,858,411 and 3,016,447, and other so-called "high velocity" plasma or "hypersonic" combustion spray processes.
Although coatings of W-Co-Cr-C have been employed successfully in many industrial applications over the past decade or more, there is an ever increasing demand for even better coatings having superior toughness and strength. In the petrochemical industry, for example, there is a need for special coatings of this type for use on gate valves employed in deep well service equipment for handling highly corrosive fluids under hydraulic pressures exceeding 10,000 psi.
As is generally known, coatings of W-Co-Cr-C derive their toughness and strength from the presence of cobalt and their wear resistance from the formation of complex carbides of W, Co and Cr. Corrosion resistance is related to the amount of chromium employed in the coating. However, an excessive amount of chromium tends to decrease the toughness of the coating and should be avoided.
It is also known that the wear resistance of these coatings will generally increase with an increase in the amount of carbon and/or chromium employed in the coating. On the contrary, however, it is known as well that wear resistance tends to decrease with any increase in the cobalt content. A typical coating composition is therefore selected as a compromise to provide good wear resistance with adequate toughness and strength for many applications.